The present invention relates to developers for use in electrophotography.
Electrophotographic processes generally comprise the following steps: a charging step for forming a uniform charge layer on the surface of a photoreceptor; an exposure step in which an electrostatic latent image is formed by imagewise exposure; a development step for rendoring the electrostatic latent image visible by a colored toner; a transfer step for transferring the resulting toner image onto a recording member, typically paper; and a fixing step in which the transferred image is fixed as a copy image either with heat or under pressure.
Electrophotographic photoreceptors include a selenium photoreceptor, a zinc oxide photoreceptor, a cadmium sulfide photoreceptor, organic photoreceptors, an amorphous silicon photoreceptor, etc. Organic photoreceptors can not only be produced at low cost but they also have the advantages of high sensitivity, durability and heat resistance plus non-toxicity. With the recent expansion of the use of organic photoreceptors, a need has arisen for producing positively chargeable toners. In fact, however, the electrostatic latent image formed on the surface of common photoreceptors which have light-sensitive layers made of selenium and other inorganic materials has positive polarity and is rendered visible with developers having negatively chargeable toners. Thus, extensive R&D efforts have been made on developers having negatively chargeable toners but not on developers having positively chargeable toners which are useful in the development of organic photoreceptors. Under these circumstances, no satisfactory developers have yet been obtained that have positively chargeable toners.
Two types of developers are known that are useful in dry development: the first type is a one-component developer which is solely composed of a magnetic toner containing a magnetic material, and the second type is a two-component developer composed of a non-magnetic toner which is free from a magnetic material and a magnetic carrier.
In the absence of carriers, the one-component developer allows the toner particles to be electrified either by friction between themselves or by friction with the development sleeve in the developing unit or the blade for restricting the height of developer layer. As a result, toner particles charged positively intermingle with negatively charged ones and this fact, combined with the small quantity of triboelectrification, tends to make development instable. On the other hand, the two-component developer consists of a toner and a carrier, with the carrier having a capability for permitting the toner to be charged in a desired polarity. Thus, toner particles can be provided with triboelectric charges of appropriate polarity in an appropriate amount. In other words, the two-component developer is capable of by far improved triboelectrification over the one-component developer. Another advantage of the two-component developer is that the amount of charges on the toner can be controlled over a satisfactorily broad range by selecting a carrier having desired characteristics.
However, in order to obtain a final fixed image of good quality, efficient triboelectrification of the developer does not suffice and it is also necessary that the developer particles provided with triboelectric charges in the developing unit be transported in the developing space without agglomeration. Speaking of magnetic brush development, it is necessary that the developer provided with triboelectric charges by agitation in the developing unit be held on the development sleeve like a high-pile blanket in which the particles are uniformly aligned into thistles and that such a development layer be transported consistently through the developing space with the blanket being maintained.
If toner particles in a two-component developer are highly likely to agglomerate into lumps by an electrostatic cohesive force, it is difficult to disperse them in carrier particles at uniform concentration and the proportion of toner particles provided with a smaller quantity of triboelectric charges will decrease on account of reduced friction between toner and carrier particles. As a result, toner particles will be deposited on the non-image areas of the photoreceptor during development, producing a fogged final image after fixing. Further, the presence of many toner particles that are weakly charged reduces the force of adhesion between toner and carrier particles and in magnetic brush development, toner particles that are being transported through the developing space will spin off the magnetically rotating carrier particles under centrifugal force. As a consequence, the charging device, the exposure optical system and other units in the copying machine will be fouled by the toner particles, causing defects such as unevenness and clear spots in the finally obtained fixed image.
In conventional toners of a negatively chargeable type, silica particles finer than toner particles are mixed with the latter so that they are deposited on the surfaces of toner particles to prevent agglomeration of the latter and to insure high fluidity. However, the conventionally used fine silica particles have such a strong tendency to be negatively charged that if they are mixed with toner particles so as to be deposited on the surfaces of the latter, the resulting tonor will have negative, rather than positive, chargeability. As a result, the toner has the same polarity as that of the negatively charged electrostatic latent image formed on the photoreceptor, thus making it impossible to effect electrostatic development.
With a view to solving these problems, the following techniques have been proposed:
(1) using positively chargeable fine particles treated with a silane coupling agent (see Unexamined Published Japanese Patent Application Nos. 53-66235 and 56-123550, and Examined Japanese Patent Publication No. 53-22447); and
(2) using positively chargeable fine particles treated with silicone oil (see Unexamined Published Japanese Patent Application Nos. 58-60754 and 59-187359).
However, charging toners positively does not suffice for image of good quality to be maintained consistantly for a prolonged period of time. Toner is subjected to mechanical agitation in the developing unit in order to achieve its electrification through friction with the surface of carrier particles. If the toner is put to cyclic use, part of the toner components will stick to the surface of carrier particles (this phenomenon is hereinafter referred to as "toner loss"). In case of a resin-coated carrier, the resin forming the carrier surface will gradually wear to reduce the carrier's surface area which is effective in imparting an appropriate amount of triboelectric charges. In either case, the amount of triboelectrification on the toner decreases to increase the chance of toner particles of flying about in the copying machine or of the occurrence of fogging.
With the recent demand for reducing the size, power consumption and copying cost of electrophotographic copiers, a developer is desired that need only be used in a small amount and which yet is capable of producing image of good quality in a consistent way over a prolonged period. In other words, a toner is desired that can be provided with a sufficient amount of triboelectrification to produce image of good quality even if the surface area of carrier is reduced on account of using the developer in a smaller amount.